Prior drop-on-demand ink jet printers typically employ one or more inks of a single intensity. Images are formed on a recording medium by ejecting drops of ink from an ink jet head onto the medium. Color ink jet printers typically use four subtractive primary colors of ink: cyan, magenta, yellow and black. Non-primary colors are produced by printing dots of different subtractive primary colors on top of one another. Modulation of the intensity of color of the printed image, hereinafter referred to as gray scale printing, is typically achieved by one of two methods: (1) modulating the diameter or size of each ink dot while leaving the number of dots within a specific area of the image unchanged; or (2) varying the number of dots printed in a specific area without changing the diameter of each individual dot.
Modulation of ink dot size entails controlling the volume of each drop of ink ejected by the ink jet head. The larger the dot size, the darker the color intensity of the printed image. Methods for modulating the volume of ink drops ejected from an ink jet print head are known in the art. For example, U.S. Pat. No. 3,946,398 describes a drop-on-demand ink jet print head that ejects ink drops of variable size in response to pressure pulses developed in an ink pressure chamber by a piezoceramic transducer (PZT). Ink drop volume is modulated by varying the amount of electrical waveform energy applied to the PZT for the generation of each pressure pulse. However, varying the ink drop volume causes variation in the ink drop ejection velocity resulting in drop landing position errors.
U.S. Pat. No. 4,393,384 describes a method for independently controlling both the drop volume and ejection velocity. In order to provide dots small enough for low intensity images, a very small ink jet orifice is required. Such an ink jet print head is difficult to manufacture and clogs easily.
Other approaches have employed a method for controlling the drop volume size and the drop ejection velocity by means of an electric field which accelerates the ink drops in inverse proportion to their volume, thereby reducing the effect of variations in ejection velocity. In addition, the electric field enables formation of an ink drop smaller than the orifice diameter. However, use of the electric field increases the complexity and cost of the printer.
U.S. Pat. No. 5,495,270, issued Feb. 27, 1996 and assigned to the assignee of the present application, discloses an ink jet printer which produces ink drops of differing volumes having substantially the same ejection velocity by providing multiple PZT drive waveforms. The number of different ink drop sizes and therefore the number of gray scale levels which can be produced using this technique is very limited. In addition, the technology required to implement this method is quite complex.
In single ink dot size printing, the printer provides drops of one size which are large enough to provide adequate "solid fill" printing for a given resolution. Color intensity is manipulated by a process referred to as "dithering" in which the perceived intensity of an array of dots is modulated by selectively printing or not printing individual dots within an array thereby varying the number of dots in the specific area. For example, if a 50 percent average intensity is desired, half of the dots in the array are printed. Multiple dither pattern dot densities are possible to provide a wide range of intensity levels. For a two-by-two dot array, five intensity level patterns are possible. An eight-by-eight dot array can produce 65 different intensity levels. Usable gradations of color in an image are thus achieved by distributing a myriad of appropriately dithered arrays across the recording medium in a predetermined arrangement.
However, with dithering there is a trade-off between the number of possible intensity levels and the size of the dot array required to achieve those levels. Increasing the size of the dither cell leads to loss of spatial accuracy due to the lower resolution of the dither patterns. This in turn results in printed images having a grainy appearance.
The Canon FP-510 printer employs ink drops of varying sizes to produce an image of varying color intensity. The Canon FP-510 also uses three different densities of liquid, water soluble cyan and magenta ink (thick, medium and light) to provide up to 64 color gradations. In addition to using liquid ink, the Canon FP-510 can be used only with specially coated roll paper, thereby limiting the versatility of the machine.
In medical diagnostic imaging there is the need for creating images of substantial contrast between the imaged and non-imaged areas and to highlight the differences between different levels of gray obtained when using solid or phase change ink. There still remains the necessity, while using inks of varying intensity to achieve multiple gray scale levels, for long shelf life, and resistance to light.
There thus continues to be a need in the art for a simple, inexpensive and easy-to-use ink jet printer and a method of printing which provides high-resolution gray scale printing, especially in medical diagnostic imaging applications on transparency films, without sacrificing performance and versatility of use.